The present invention relates to a process for preparing perpropionic acid which is used for, e.g., epoxidation, hydroxylation, formation of lactone, formation of quinone, ring opening of an aromatic ring, formation of phenols, oxidation of a ketone and the like. More specifically, it relates to a process for preparing perpropionic acid by reacting propionic acid with hydrogen peroxide in an alkyl propionate as a reaction solvent which is feasible to form a heterogeneous azeotrope with water in the presence of a catalyst, while continuously removing water introduced with the hydrogen peroxide and water produced during the reaction by azeotropic distillation with the reaction solvent, and keeping a concentration of a peroxide in an aqueous phase separated from liquid distillated by the azeotropic distillation not more than 0.1% by weight.
When perpropionic acid is prepared by a reaction of propionic acid with hydrogen peroxide in the presence of an appropriate catalyst, it is necessary to carry out the reaction while removing (a) the water introduced with the hydrogen peroxide for shifting the reaction, which is an equilibrium reaction, to a system for formation and (b) the water produced during the reaction. It has been proposed as the known prior art to continuously remove the water by azetropic distillation in the presence of "an inert organic solvent which may form a heterogeneous azeotropic mixture with water (hereinafter, merely abbreviated as a "reaction solvent")", for example, an organo-chlorine type solvent such as 1,2-dichloroethane, 1,2-dichloropropane, chloroform, carbon tetrachloride, dichloromethane or the like, or a hydrocarbon type solvent such as benzene, toluene, cyclohexane or the like (see, e.g., Japanese Provisional Patent Publication No. 160313/1979, Japanese Patent Publication No. 64425/1988, Japanese Patent Publication No. 64426/1988, U.S. Pat. No. 2,877,266, U.S. Pat. No. 2,814,641, etc.).
However, the aforementioned methods involve the following defects:
(1) When the water is continuously removed by azeotropic distillation in the reaction in progress, a considerable amount of the peroxides are distilled in the form of unaltered hydrogen peroxide and/or perpropionic acid with an aqueous phase to be removed by distillation to cause a loss of the peroxide. Moreover, in a distillation column where a stabilizer added to the reaction system substantially does not exert the effect, the peroxides will be decomposed to cause a loss. Accordingly, a conversion rate of the hydrogen peroxide and selective reactivity to perpropionic acid are lowered to be unsatisfactory for the utilization in industry.
(2) Where an organo-chlorine type solvent such as 1,2-dichloroethane, 1,2-dichloropropane etc. is used as a reaction solvent, the use of austenitic stainless steel generally used as a material for apparatus may cause troubles such as stress corrosion.
(3) When an organic chlorine type solvent such as 1,2-dichloroethane is used, a catalyst such as a boric acid type catalyst separates out so much times after completion of the reaction. It is necessary to filter and separate the catalyst and the apparatus becomes complex. There is also a problem that the danger due to the adsorption of peroxides is increased.
Further, as a process for solving the aforementioned defect (1), there has been proposed to prevent decomposition of the peroxide and the loss of the peroxide to the aqueous phase by injecting continuously water and propionic acid into a distillation column from the head for inhibiting the rise of the peroxide in the distillation column (for example, Japanese Provisional Patent Publication No. 113173/1983, Japanese Provisional Patent Publication No. 159365/1988, etc.).
Although this method is excellent in view of improvement of a conversion rate of hydrogen peroxide and selective reactivity to produce the perpropionic acid, the following problems still remain unsolved.
That is, where water is injected into the distillation column from the head, it is necessary to further remove the injected water in addition to the water introduced with hydrogen peroxide and the water produced in the reaction resulting in problems a disadvantageous energy consumption and the need for a complicated apparatus.
Also, where propionic acid is injected from the head of column, in addition to the above problem, there is problem that, for example, when .epsilon.-caprolactone is prepared by using perpropionic acid as the product, the amount of propionic acid is gradually increased with respect to a reaction solvent and whereby separation of the reaction solvent from the propionic acid is required at some point of time in view of recycling of the propionic acid and reaction solvent.
As for the aforementioned defects (2) and (3), it is possible to solve them by purifying the reaction solvent and using a high grade stainless steel. However, it is not a satisfactory process in industry due to the complexity of the apparatus.